The present invention relates to a sewing machine having a cylindrical base, more particularly, a sewing machine having a U-shaped base housing with one or two feed dog advancement bars which are driven in an alternating longitudinal movement parallel to the axis of the base and in an alternating lifting movement about a pin disposed perpendicular to this axis. The combined action of these two movements serve to advance a workpiece along the base.
In conventional sewing machines, the cylindrical base comprises a substantially U-shaped cross section, which is closed at its upper part by a cover which constitutes the workpiece support surface.
In the aforementioned sewing machine base, the feed dog support bars perform their function between the internal walls and are guided in their movement by portions of these walls, which have been precision-worked.
For this reason, it is obviously necessary to thoroughly lubricate the internal walls and owing to the fact that the base is relatively long, the cover for the upper closure is long, thus resulting in the disadvantage of lubricant escaping at the connection zone between the cover and the surface on which this cover is disposed. In this type of machine, the workpiece is generally in contact with the zone from which the lubricant is escaping, and there is the disadvantage that the workpiece will be soiled relatively easily. A further disadvantage consists in that accurate and thus costly mechanical working is required to prevent leakage of lubricant between the cover and the base.
Another disadvantage results from the fact that, owing to the inevitable wear between the feed dog support bars and the guide walls, it is necessary to replace these bars when the play becomes excessive for the walls in joint are an integral part of the base.